the regex was intended to be used in the constraint column as part of a regex() function validating the pattern. See the 'Regular Expressions' subsection in Formulas—Survey123 for ArcGIS | ArcGIS for more information.

The evaluator '{A-Z][a-z][0-9]+' has an issue - the first brace is a curly brace instead of a square brace. Square braces are used to group sets of characters together - [A-Z] looks for an uppercase letter, [a-z] looks for a lower case letter, [0-9] looks for a numeric digit. The pattern you gave (Aa0 ) indicates that these should be grouped together using (); I also included a space as that was what you typed initially (you could also use \s instead of the space, though that would match other whitespace characters):

([A-Z][a-z][0-9] )

You wanted to match at least 1 and possibly more repetitions of this, the + (or * for 0 or more) is used to repeat a pattern:

([A-Z][a-z][0-9] )+

To ensure that the answer has nothing but this pattern, we should also specify to start the match at the beginning (^) and only finish at the end ($)